Calcium citrate, calcium with vitamin K or oyster shell? Drugstore shelves offer a variety of calcium supplements for strong bones, plus added nutrients like vitamins D, K and magnesium. What do you really need? Here’s how to choose the best calcium supplement for you...

In a perfect world, we’d get all the nutrients to build strong bones from foods like dairy products, leafy greens and fatty fish.

But most women get only 50% of the calcium and 40% or less of the bone-building vitamin D they need each day (about 300 IUs), according to the National Institutes of Health (NIH). Does that mean most of us should take supplements?

According to the Institute of Medicine, to build strong bones, women under age 51 need 1,000 milligrams (mg) of calcium a day and 600 IUs of vitamin D. If over 51, they need 1,200 mg of calcium. They also need 320 mg of magnesium (which helps turn calcium into bone) and 90 micrograms (mcg) of vitamin K (which helps cells absorb calcium).

But not all supplements are created equal. Some aren’t absorbed by the body easily, cause stomach upset, can contain dangerous heavy metals or can’t be taken with certain foods or medications.

Always check with your doctor before starting a supplement regimen and to determine the right dosage for you. Also, ask for a blood test to determine how much vitamin D you’ll also need.

Here, we asked nutrition experts to recommend the best calcium supplement for women of every age.

1. Calcium citrate Calcium citrate is the most easily absorbed form of calcium. It can be taken with or without food and usually doesn’t cause stomach upset or gas, a common problem with other types of calcium supplements.

It’s also less likely to cause constipation, unlike calcium carbonate.

Who should take it: This is the best calcium supplement for women who have trouble absorbing other types of calcium. These include women over 50 with low stomach acid (a natural part of aging), says registered dietitian Dee Sandquist, M.S., spokeswoman for the American Dietetic Association.

Calcium citrate is also a good choice for women who take acid blockers for indigestion, acid reflux or other gastrointestinal conditions, and it’s safe for women with a high risk of kidney stones, says Tod Cooperman, M.D., president of ConsumerLab.com, an independent research and testing firm.

Watch out:Calcium citrate contains half the elemental calcium as the more commonly available calcium carbonate. You’ll have to take more, or bigger tablets to get the same amount of the mineral, says Dr. Cooperman.

It’s also nearly twice as expensive as calcium carbonate, or about $10-$14 for 120 capsules. And you may still need to take vitamin D supplements to meet the recommended daily dosage of 600 IU.

2. Calcium carbonate Calcium carbonate – found in antacids like TUMS or Rolaids – has the highest amount of elemental calcium, according to the Mayo Clinic. It’s cheap – as little as $5 or so for a bottle of 120 capsules – and should be taken with meals for best absorption.

Who should take it: This is the best calcium supplement forwomen under 50 looking for an inexpensive calcium supplement. But it won’t help you meet your 600 mg of RDA for vitamin D.

Watch out:Calcium carbonate needs an acidic atmosphere to be absorbed through the stomach, so it’s not a good choice if you’re over 50 or take acid blockers.

TUMS, Rolaids and other antacids with calcium carbonate contain only 200 mg of elemental calcium per tablet, says Dr. Cooperman, so don’t depend on them to meet your daily calcium needs.

In fact, popping a handful of TUMS may not be a smart way to get the RDA of calcium, according to a 2007 study at Mercer University School of Medicine in Georgia.

Researchers found a link between excessive antacid consumption and milk-alkali syndrome, a condition that causes calcium deposits in the kidneys and other tissues, which may lead to kidney failure.

Calcium carbonate is also the type found in oyster shell and coral calcium. But it’s more expensive, environmentally unfriendly, and may cause serious reactions in people allergic to seafood, says Dr. Cooperman. Some brands even contain dangerous levels of lead.

The NIH, Consumer Reports, Mayo Clinic, Food and Drug Administration (FDA) and the Federal Trade Commission have all charged manufacturers of one brand (Coral Calcium Supreme) with making false and unsubstantiated claims on its curative powers.

Coral calcium costs more too – up to $26 or more per bottle (60 tablets).

Sunlight is the best way to get vitamin D – just 10 minutes of exposure can help prevent deficiencies, says Michael Holick, Ph.D., M.D., professor of medicine at Boston University and author of The Vitamin D Solution(Hudson Street Press).

But many women don’t get enough. If you limit your time outside or wear sunscreen religiously, you could be vitamin D deficient.

“Taking a daily calcium supplement with vitamin D is the easiest way to make sure you’re getting what you need for bone health,” Dr. Holick says.

Typical calcium-with-D supplements contain 400 IUs of vitamin D per pill, not enough to meet the 600 IU requirement, and you might need to take an extra pill.

Who should take it:This is best calcium supplement for women over 70 (your skin produces less vitamin D as you age) and those who wear sunscreen, have dark skin, or live in mostly cloudy or rainy climates. See your doctor to get a blood test to determine your vitamin D levels.

Watch out: Vitamin D is good for you – but you can get too much of a good thing.

“Adults can safely take 10,000 IU of vitamin D daily with no ill side effects,” Dr. Holick says.

But taking more than that over time may cause hypervitaminosis D. Symptoms include nausea, vomiting, poor appetite, constipation, weakness, confusion, heart rhythm abnormalities and kidney stones.

But most women don’t need a calcium magnesium supplement combo. That’s because magnesium and vitamin K are abundant in many foods, like fish and nuts, and most healthy eaters easily consume sufficient doses in their diet.

You can get more than your daily quota of magnesium (320 mg) by eating a 3-ounce serving of halibut (90 mg), 2 ounces of almonds (160 mg) and a cup of soybeans (150 mg), according to the NIH.

It’s even easier to meet the 90 mcg quota for vitamin K. A cup of broccoli has 220 mcg; kale 547 mcg per cup.

Vitamin K is safe in high doses and could even prevent fractures.

According to the 1999 landmark Nurse’s Health Study that followed 72,000 women for 10 years, women with a very low vitamin K intake had a 30% higher risk of hip fractures.

A 2007 Japanese study found that women taking 10 times the RDA for vitamin K had higher bone density and a lower risk of fracture.

Who should take it:Calcium magnesium supplements benefit women with chronic diarrhea or diseases like diabetes, cystic fibrosis, Crohn's or celiac, which inhibit magnesium absorption in the body.

Also, alcoholics, or those undergoing chemotherapy or taking diuretics, like Lasix, are more likely to be deficient in magnesium, according to the Linus Pauling Institute.

You’re also a good candidate for vitamin K/calcium magnesium supplement combination if you take broad-spectrum antibiotics for prolonged periods, such as drugs in the sulfonamide, tetracycline and penicillin families, according to the Mayo Clinic.

Watch out: Avoid vitamin K and calcium combination supplements if you take anticoagulants like warfarin. The vitamin can inhibit the drug’s ability to clot blood.

The best calcium supplement for your buckLook for the USP symbol: Several studies have found dangerous levels of lead in calcium carbonate supplements made from bones, shells, bone meal, oyster shell and dolomite.

Most manufacturers have reduced lead to safe levels, according to further studies in 2008 by Consumerlab.com. To make sure a supplement is lead-free, look for the USP (United States Pharmacopeia) symbol, which certifies that the supplement has no dangerous metals. Even small amounts of lead can devastate your nervous system.

The USP symbol also guarantees quality, purity and potency and ensures it will dissolve in your stomach.

Divvy up your daily dose: Your body can absorb only about 500 mgs of calcium at a time, according to a 2007 review of calcium doses and forms, published in the journal Nutrition in Clinical Practice. Split up your daily dose and take it 2-3 times daily to maximize absorption.

Time your meds: Take calcium supplements two hours apart fromantibiotics in the tetracycline or fluoroquinolone families (Cipro, Floxin, Noroxin) as well as thyroid medications (Synthroid, Armour Thyroid, Cytomel, Nature-throid). Calcium interferes with the body’s absorption of antibiotics, according to Helen Hayes Hospital in New York, which conducts extensive research on vitamin supplements.

Don’t take iron with calcium:Take iron two hours before or after you take calcium supplements, so both get absorbed efficiently, according to Helen Hayes Hospital, a member of the New York-Presbyterian Healthcare System.

Ask your doctor:Don’t take calcium or any other supplements without consulting your doctor.

Don’t overdo it:Studies show that more than 2,500 mg of calcium daily can lead to rare side effects, including increased risk of kidney stones, kidney failure and hypercalcemia (too much calcium in the blood), according to the 2007 Canyon Ranch study.

A July 2010 study in the British Medical Journal linked calcium supplements without vitamin D with a modest increase in cardiovascular disease, but has recommended further study.

What’s Your Supplement IQ?You know that taking calcium supplements can help build strong bones when you don't eat enough dairy foods, but do you really know all that you should about supplements? Beyond the world of basic nutrition, there's a different solar system of weird, wacky and wonderful facts about vitamins, minerals and herbs. Here's your chance to find out how much you know: Test your IQ with this supplement quiz.

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